diff --git a/CHANGES.md b/CHANGES.md
index 9ca9650..2f341c3 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,3 +1,12 @@
+# 0.4.4
+### new features
+    - add support for passing se_wcs to make_exp function in sim.py
+    - enable user to define the world origin of the layout when defining galaxy
+      catalog
+    - add option to force the center of coadd boundary box to be at
+      world_origin (simple_coadd_bbox=True)
+
+
 ## 0.4.3
 ### new features
     - add support for ring test
diff --git a/descwl_shear_sims/constants.py b/descwl_shear_sims/constants.py
index 144f6bd..231659a 100644
--- a/descwl_shear_sims/constants.py
+++ b/descwl_shear_sims/constants.py
@@ -32,3 +32,7 @@
     ra=200 * galsim.degrees,
     dec=0 * galsim.degrees,
 )
+
+# When drawing galaxies, exclude those with centers falling
+# SIM_INCLUSION_PADDING away from the single exposure boundaries
+SIM_INCLUSION_PADDING = 200
diff --git a/descwl_shear_sims/galaxies.py b/descwl_shear_sims/galaxies.py
index 3d6688b..89e16a6 100644
--- a/descwl_shear_sims/galaxies.py
+++ b/descwl_shear_sims/galaxies.py
@@ -25,7 +25,7 @@ def make_galaxy_catalog(
     coadd_dim=None,
     buff=0,
     pixel_scale=SCALE,
-    layout=None,
+    layout: Layout | str | None = None,
     gal_config=None,
     sep=None,
 ):
@@ -642,7 +642,7 @@ def __init__(
         self,
         *,
         rng,
-        layout='random',
+        layout: Layout | str = 'random',
         coadd_dim=None,
         buff=None,
         pixel_scale=SCALE,
diff --git a/descwl_shear_sims/layout/layout.py b/descwl_shear_sims/layout/layout.py
index 36266e1..3f0dff2 100644
--- a/descwl_shear_sims/layout/layout.py
+++ b/descwl_shear_sims/layout/layout.py
@@ -4,6 +4,7 @@
     GRID_SPACING,
     HEX_SPACING,
     SCALE,
+    WORLD_ORIGIN,
 )
 from .shifts import (
     get_pair_shifts,
@@ -12,7 +13,7 @@
     get_random_shifts,
     get_random_disk_shifts,
 )
-from ..wcs import make_coadd_dm_wcs
+from ..wcs import make_coadd_dm_wcs, make_coadd_dm_wcs_simple
 import numpy as np
 
 
@@ -23,9 +24,14 @@ def __init__(
         coadd_dim=None,
         buff=0.0,
         pixel_scale=SCALE,
+        world_origin=WORLD_ORIGIN,
+        simple_coadd_bbox=False,
     ):
         """
         Layout object to make position shifts for galaxy and star objects
+        The scale of the layout is coadd_dim * pixel_scale. The shifts is
+        defined on coadd image (flat sky) with repect to the center of coadd
+        boundary box.
 
         Parameters
         ----------
@@ -36,11 +42,21 @@ def __init__(
         buff: int, optional
             Buffer region where no objects will be drawn.  Default 0.
         pixel_scale: float
-            pixel scale
+            pixel scale of coadd image
+        world_origin: galsim.CelestialCoord
+            sky coordinate of the reference point (sky coordinate of the center
+            of large box)
+        simple_coadd_bbox: bool. Default: False
+            If set to True, the coadd boundary box is centered at world_origin;
+            that is, the center of the coadd boundary box is the image origin;
+            else the center of the coadd boundary box has an offset to the
+            world_orgin, and it is not the image origin
         """
         self.pixel_scale = pixel_scale
         self.layout_name = layout_name
         if layout_name == 'random':
+            if coadd_dim is None:
+                raise ValueError("Please input `coadd_dim` for random layout")
             # need to calculate number of objects first this layout is random
             # in a square
             if (coadd_dim - 2*buff) < 2:
@@ -50,6 +66,10 @@ def __init__(
                 # [arcmin^2]
                 self.area = ((coadd_dim - 2*buff)*pixel_scale/60)**2
         elif layout_name == 'random_disk':
+            if coadd_dim is None:
+                raise ValueError(
+                    "Please input `coadd_dim` for random_disk layout"
+                )
             # need to calculate number of objects first
             # this layout_name is random in a circle
             if (coadd_dim - 2*buff) < 2:
@@ -70,10 +90,17 @@ def __init__(
                     'hex', 'grid' or 'pair'!")
         self.coadd_dim = coadd_dim
         self.buff = buff
-        self.wcs, self.bbox = make_coadd_dm_wcs(
-            coadd_dim,
-            pixel_scale=pixel_scale,
-        )
+
+        if simple_coadd_bbox:
+            self.wcs, self.bbox = make_coadd_dm_wcs_simple(
+                coadd_dim,
+                pixel_scale=pixel_scale,
+            )
+        else:
+            self.wcs, self.bbox = make_coadd_dm_wcs(
+                coadd_dim,
+                pixel_scale=pixel_scale,
+            )
         return
 
     def get_shifts(
diff --git a/descwl_shear_sims/sim.py b/descwl_shear_sims/sim.py
index e357c02..faaf144 100644
--- a/descwl_shear_sims/sim.py
+++ b/descwl_shear_sims/sim.py
@@ -9,7 +9,7 @@
 from .lsst_bits import get_flagval
 from .saturation import saturate_image_and_mask, BAND_SAT_VALS
 from .surveys import get_survey, rescale_wldeblend_exp, DEFAULT_SURVEY_BANDS
-from .constants import SCALE, ZERO_POINT, WORLD_ORIGIN
+from .constants import SCALE, WORLD_ORIGIN, ZERO_POINT, SIM_INCLUSION_PADDING
 from .artifacts import add_bleed, get_max_mag_with_bleed
 from .masking import (
     get_bmask_and_set_image,
@@ -17,7 +17,9 @@
 )
 from .objlists import get_objlist
 from .psfs import make_dm_psf
-from .wcs import make_wcs, make_dm_wcs, make_coadd_dm_wcs, make_coadd_dm_wcs_simple
+from .wcs import (
+    make_dm_wcs, make_se_wcs, make_coadd_dm_wcs, make_coadd_dm_wcs_simple
+)
 from .shear import ShearConstant
 
 
@@ -135,14 +137,9 @@ def make_sim(
         Dimensions for planned final coadd.  This is used for generating
         the final coadd WCS and deteremines some properties of
         the single epoch images.
-    simple_coadd_bbox: optional, bool. Default False
-        If set to True, the coadd bbox is a simple box bounded by
-        (0,0,coadd_dim,coadd_dim), and the coadd and SE WCS have the same
-        world origin. If set to False, the coadd bbox is embeded in a larger
-        box bounded by (xoff, yoff, xoff+coadd_dim, yoff+coadd_dim) and
-        the SE WCS has a different world origin compared to the coadd WCS.
-        This option overrides the wcs in galaxy catalog layout and requires
-        an input coadd_dim.
+    simple_coadd_bbox: optional, bool. Default: False
+        Whether to force the center of coadd boundary box (which is the default
+        center single exposure) at the world_origin
     draw_noise: optional, bool
         Whether draw image noise
 
@@ -174,30 +171,41 @@ def make_sim(
         survey_name=survey_name,
     ).pixel_scale
 
-    if (
-        hasattr(galaxy_catalog.layout, "wcs")
-        and hasattr(galaxy_catalog.layout, "bbox")
-        and not simple_coadd_bbox
-    ):
-        coadd_wcs = galaxy_catalog.layout.wcs
-        coadd_bbox = galaxy_catalog.layout.bbox
-    else:
-        if not isinstance(coadd_dim, int):
-            raise ValueError(
-                "coadd_dim should be int when galaxy catalog does not",
-                "have attribute 'layout'",
-            )
-        if simple_coadd_bbox:
-            coadd_wcs, coadd_bbox = make_coadd_dm_wcs_simple(
-                coadd_dim,
-                pixel_scale=pixel_scale,
+    if simple_coadd_bbox:
+        # Force to use a simple coadd boundary box
+        # where the center of the boundary box is the image origin and it
+        # matches to the world origin of the catalog's layout. Note that he
+        # center of the boundary box is the image_origin of the single exposure
+        # (SE).
+        if hasattr(galaxy_catalog.layout, "wcs"):
+            origin = galaxy_catalog.layout.wcs.getSkyOrigin()
+            world_origin = galsim.CelestialCoord(
+                ra=origin.getRa().asDegrees() * galsim.degrees,
+                dec=origin.getDec().asDegrees() * galsim.degrees,
             )
+        else:
+            world_origin = WORLD_ORIGIN
+        coadd_wcs, coadd_bbox = make_coadd_dm_wcs_simple(
+            coadd_dim,
+            pixel_scale=pixel_scale,
+            world_origin=world_origin,
+        )
+    else:
+        if (
+            hasattr(galaxy_catalog.layout, "wcs")
+            and hasattr(galaxy_catalog.layout, "bbox")
+        ):
+            coadd_wcs = galaxy_catalog.layout.wcs
+            coadd_bbox = galaxy_catalog.layout.bbox
         else:
             coadd_wcs, coadd_bbox = make_coadd_dm_wcs(
                 coadd_dim,
                 pixel_scale=pixel_scale,
             )
 
+    # get the sky position of the coadd image center. For simple_coadd_bbox ==
+    # True, coadd_bbox_cen_gs_skypos is WORLD_ORIGIN (see unit test_make_exp in
+    # test_sim.py)
     coadd_bbox_cen_gs_skypos = get_coadd_center_gs_pos(
         coadd_wcs=coadd_wcs,
         coadd_bbox=coadd_bbox,
@@ -279,7 +287,6 @@ def make_sim(
                 calib_mag_zero=calib_mag_zero,
                 draw_noise=draw_noise,
                 indexes=lists["indexes"],
-                simple_coadd_bbox=simple_coadd_bbox,
             )
             if epoch == 0:
                 bright_info += this_bright_info
@@ -360,10 +367,10 @@ def make_exp(
     calib_mag_zero=ZERO_POINT,
     draw_noise=True,
     indexes=None,
-    simple_coadd_bbox=False,
+    se_wcs=None,
 ):
     """
-    Make an SEObs
+    Make an Signle Exposure (SE) observation
 
     Parameters
     ----------
@@ -422,14 +429,13 @@ def make_exp(
         rotation angle of intrinsic galaxies and positions [for ring test],
         default 0, in units of radians
     pixel_scale: float
-        pixel scale in arcsec
+        pixel scale of single exposure in arcsec
     calib_mag_zero: float
         magnitude zero point after calibration
     indexes: list
-        list of indexes in the input galaxy catalog
-    simple_coadd_bbox: optional, bool. Default False
-        If set to True, the SE WCS sky origin is forced to be WORLD_ORIGIN,
-        which is consistent with the simple coadd wcs.
+        list of indexes in the input galaxy catalog, default: None
+    se_wcs: galsim WCS
+        wcs for single exposure, default: None
     Returns
     -------
     exp: lsst.afw.image.ExposureF
@@ -445,42 +451,37 @@ def make_exp(
         ra, dec: sky position of input galaxies
         z: redshift of input galaxies
         image_x, image_y: image position of input galaxies
-    """
-    dims = [dim] * 2
-    # Galsim uses 1 offset. An array with length =dim=5
-    # The center is at 3=(5+1)/2
-    cen = (np.array(dims) + 1) / 2
-    se_origin = galsim.PositionD(x=cen[1], y=cen[0])
-    if coadd_bbox_cen_gs_skypos is None:
-        coadd_bbox_cen_gs_skypos = WORLD_ORIGIN
-    if dither:
-        dither_range = 0.5
-        off = rng.uniform(low=-dither_range, high=dither_range, size=2)
-        offset = galsim.PositionD(x=off[0], y=off[1])
-        se_origin = se_origin + offset
-
-    if rotate:
-        theta = rng.uniform(low=0, high=2 * np.pi)
-    else:
-        theta = None
+    se_wcs: galsim wcs
+        the wcs of the single exposure
 
-    # galsim wcs
-    # if simple coadd bbox, force the SE WCS to share the same
-    # world origin as the coadd WCS
-    if simple_coadd_bbox:
-        se_wcs = make_wcs(
-            scale=pixel_scale,
-            theta=theta,
-            image_origin=se_origin,
-            world_origin=WORLD_ORIGIN,
-        )
-    else:
-        se_wcs = make_wcs(
-            scale=pixel_scale,
-            theta=theta,
+    """
+    dims = [int(dim)] * 2
+
+    if se_wcs is None:
+        # If no se_wcs input, we make a wcs with world origin set to the center
+        # of the coadds (the center of the galaxy_catalog.layout)
+
+        # The se origin is set to the center of the image
+        # Galsim image uses 1 offset. An array with length =dim=5
+        # The center is at 3=(5+1)/2
+        cen = (np.array(dims) + 1) / 2
+        se_origin = galsim.PositionD(x=cen[1], y=cen[0])
+        se_wcs = make_se_wcs(
+            pixel_scale=pixel_scale,
             image_origin=se_origin,
             world_origin=coadd_bbox_cen_gs_skypos,
+            dither=dither,
+            rotate=rotate,
+            rng=rng,
         )
+    else:
+        # if with se_wcs input, we make sure the wcs is consistent with the
+        # other inputs
+        cen = se_wcs.crpix
+        se_origin = galsim.PositionD(x=cen[1], y=cen[0])
+        pixel_area = se_wcs.pixelArea(se_origin)
+        if not (pixel_area - pixel_scale ** 2.0) < pixel_scale ** 2.0 / 100.0:
+            raise ValueError("The input se_wcs has wrong pixel scale")
 
     image = galsim.Image(dim, dim, wcs=se_wcs)
 
@@ -564,7 +565,7 @@ def make_exp(
 
     # Prepare the frc, and save it to the DM exposure
     # It can be retrieved as follow
-    # zero_flux=  exposure.getPhotoCalib().getInstFluxAtZeroMagnitude()
+    # zero_flux =  exposure.getPhotoCalib().getInstFluxAtZeroMagnitude()
     # magz    =   np.log10(zero_flux)*2.5 # magnitude zero point
     zero_flux = 10.0 ** (0.4 * calib_mag_zero)
     photoCalib = afw_image.makePhotoCalibFromCalibZeroPoint(zero_flux)
@@ -658,17 +659,23 @@ def _draw_objects(
         )
         prelensed_image_pos = wcs.toImage(prelensed_world_pos)
 
-        local_wcs = wcs.local(image_pos=image_pos)
-
-        convolved_object = get_convolved_object(obj, psf, image_pos)
+        if (
+            (image.bounds.xmin - SIM_INCLUSION_PADDING) <
+            image_pos.x < (image.bounds.xmax + SIM_INCLUSION_PADDING)
+        ) and (
+            (image.bounds.ymin - SIM_INCLUSION_PADDING)
+            < image_pos.y < (image.bounds.ymax + SIM_INCLUSION_PADDING)
+        ):
+            local_wcs = wcs.local(image_pos=image_pos)
+            convolved_object = get_convolved_object(obj, psf, image_pos)
+            stamp = convolved_object.drawImage(
+                center=image_pos, wcs=local_wcs, method=draw_method, **kw
+            )
 
-        stamp = convolved_object.drawImage(
-            center=image_pos, wcs=local_wcs, method=draw_method, **kw
-        )
+            b = stamp.bounds & image.bounds
+            if b.isDefined():
+                image[b] += stamp[b]
 
-        b = stamp.bounds & image.bounds
-        if b.isDefined():
-            image[b] += stamp[b]
         info = get_truth_info_struct()
 
         info["index"] = (ind,)
diff --git a/descwl_shear_sims/surveys.py b/descwl_shear_sims/surveys.py
index 8a845a0..3431c31 100644
--- a/descwl_shear_sims/surveys.py
+++ b/descwl_shear_sims/surveys.py
@@ -8,6 +8,7 @@
     "DES": "r",
     "Euclid": "VIS",
     "CFHT": "i",
+    "Roman": "H158",
 }
 
 
diff --git a/descwl_shear_sims/tests/test_dmwcs.py b/descwl_shear_sims/tests/test_dmwcs.py
index 876d1c4..337929d 100644
--- a/descwl_shear_sims/tests/test_dmwcs.py
+++ b/descwl_shear_sims/tests/test_dmwcs.py
@@ -2,7 +2,7 @@
 import galsim
 import lsst.afw.image as afw_image
 import lsst.geom as geom
-from ..wcs import make_wcs, make_dm_wcs, make_coadd_dm_wcs_simple
+from ..wcs import make_wcs, make_se_wcs, make_dm_wcs, make_coadd_dm_wcs_simple
 from ._wcs import make_sim_wcs, SCALE
 from ..sim import get_coadd_center_gs_pos
 
@@ -160,3 +160,46 @@ def test_same_world_origin_se_coadd_wcs_simple():
         se_sky_origin.getDec().asRadians(),
         coadd_sky_origin.getDec().asRadians(),
     )
+
+
+def test_make_se_wcs():
+    se_dim = 30
+    coadd_dim = 20
+
+    dims = [se_dim] * 2
+    # Galsim uses 1 offset. An array with length =dim=5
+    # The center is at 3=(5+1)/2
+    cen = (np.array(dims) + 1) / 2
+    se_origin = galsim.PositionD(x=cen[1], y=cen[0])
+
+    masked_image = afw_image.MaskedImageF(coadd_dim, coadd_dim)
+    exp = afw_image.ExposureF(masked_image)
+
+    tcoadd_dm_wcs_simple, coadd_bbox = make_coadd_dm_wcs_simple(coadd_dim, SCALE)
+
+    exp.setWcs(tcoadd_dm_wcs_simple)
+
+    dm_coadd_wcs_simple = exp.getWcs()
+
+    coadd_bbox_cen_gs_skypos = get_coadd_center_gs_pos(
+        coadd_wcs=dm_coadd_wcs_simple,
+        coadd_bbox=coadd_bbox,
+    )
+
+    wcs = make_wcs(
+        scale=SCALE,
+        theta=0,
+        image_origin=se_origin,
+        world_origin=coadd_bbox_cen_gs_skypos,
+    )
+
+    se_wcs = make_se_wcs(
+        pixel_scale=SCALE,
+        theta=0,
+        image_origin=se_origin,
+        world_origin=coadd_bbox_cen_gs_skypos,
+        rotate=False,
+        dither=False,
+    )
+
+    assert wcs == se_wcs
diff --git a/descwl_shear_sims/tests/test_sim.py b/descwl_shear_sims/tests/test_sim.py
index 3e0815c..9f5f89f 100644
--- a/descwl_shear_sims/tests/test_sim.py
+++ b/descwl_shear_sims/tests/test_sim.py
@@ -1,17 +1,23 @@
 import os
+import galsim
 import pytest
 import numpy as np
 from copy import deepcopy
 import lsst.afw.image as afw_image
 import lsst.afw.geom as afw_geom
+
+from descwl_shear_sims.layout.layout import Layout
 from ..surveys import get_survey, DEFAULT_SURVEY_BANDS
 
 from ..galaxies import make_galaxy_catalog, DEFAULT_FIXED_GAL_CONFIG
 from ..stars import StarCatalog, make_star_catalog
 from ..psfs import make_fixed_psf, make_ps_psf
+from ..wcs import make_se_wcs
 
-from ..sim import make_sim, get_se_dim
-from ..constants import ZERO_POINT
+from ..sim import (
+    make_sim, make_exp, get_se_dim, get_coadd_center_gs_pos, get_objlist
+)
+from ..constants import SCALE, ZERO_POINT, WORLD_ORIGIN
 
 from ..shear import ShearConstant
 
@@ -701,6 +707,95 @@ def test_sim_truth_info():
     )
 
 
+def test_make_exp():
+    """
+    Unit test for make_exp
+    """
+
+    # Random number generator
+    seed = 74321
+    rng = np.random.RandomState(seed)
+
+    dim = 400
+    dims = [int(dim)] * 2
+    pixel_scale = SCALE
+    psf_dim = 51
+
+    # Define a Layout on tagent plane
+    # the size of the plane is SCAL * dim = 80 arcsec
+    # The world origin of the tangent plane is set to WORLD_ORIGIN
+    layout = Layout(
+        "grid",
+        coadd_dim=dim,
+        buff=0.0,
+        pixel_scale=SCALE,
+        world_origin=WORLD_ORIGIN,
+        simple_coadd_bbox=True,
+    )
+    galaxy_catalog = make_galaxy_catalog(
+        rng=rng,
+        gal_type="fixed",
+        layout=layout,
+    )
+    # Define the Survey
+    band = "r"
+    survey = get_survey(
+        gal_type=galaxy_catalog.gal_type,
+        band=band,
+        survey_name="lsst",
+    )
+    lists = get_objlist(
+        galaxy_catalog=galaxy_catalog,
+        survey=survey,
+    )
+
+    # assert that the world orgin is WORLD_ORIGIN
+    # NOTE: this is not the case for simple_coadd_bbox=False
+    world_origin = get_coadd_center_gs_pos(
+        coadd_wcs=galaxy_catalog.layout.wcs,
+        coadd_bbox=galaxy_catalog.layout.bbox,
+    )
+    np.testing.assert_allclose(
+        world_origin.ra.deg,
+        WORLD_ORIGIN.ra.deg,
+    )
+    np.testing.assert_allclose(
+        world_origin.dec.deg,
+        WORLD_ORIGIN.dec.deg,
+    )
+
+    # Define a PSF model for the simulation
+    psf = make_fixed_psf(psf_type="gauss")
+
+    # Define WCS for single exposure
+    # image origin for single exposure
+    cen = (np.array(dims) + 1) / 2
+    se_origin = galsim.PositionD(x=cen[1] - 100, y=cen[0] + 100)
+    se_wcs = make_se_wcs(
+        pixel_scale=pixel_scale,
+        image_origin=se_origin,
+        world_origin=world_origin,
+        dither=False,
+        rotate=False,
+        rng=rng,
+    )
+
+    exp, this_bright_info, this_truth_info, this_se_wcs = make_exp(
+        rng=rng,
+        band=band,
+        noise=0,
+        objlist=lists["objlist"],
+        shifts=lists["shifts"],
+        redshifts=lists["redshifts"],
+        dim=dim,
+        se_wcs=se_wcs,
+        psf=psf,
+        psf_dim=psf_dim,
+        shear_obj=shear_obj,
+        coadd_bbox_cen_gs_skypos=world_origin,
+    )
+
+
 if __name__ == '__main__':
     # test_sim_layout("hex", "wldeblend")
     # for rotate in (False, True):
diff --git a/descwl_shear_sims/wcs/__init__.py b/descwl_shear_sims/wcs/__init__.py
index 603b2eb..6d85272 100644
--- a/descwl_shear_sims/wcs/__init__.py
+++ b/descwl_shear_sims/wcs/__init__.py
@@ -1,3 +1,4 @@
 # flake8: noqa
 from .wcstools import make_wcs, make_coadd_wcs
 from .dmwcs import make_dm_wcs, make_coadd_dm_wcs, make_coadd_dm_wcs_simple
+from .sewcs import make_se_wcs
diff --git a/descwl_shear_sims/wcs/dmwcs.py b/descwl_shear_sims/wcs/dmwcs.py
index 98e6e07..657d106 100644
--- a/descwl_shear_sims/wcs/dmwcs.py
+++ b/descwl_shear_sims/wcs/dmwcs.py
@@ -69,9 +69,12 @@ def make_dm_wcs(galsim_wcs):
     return stack_wcs
 
 
-def make_coadd_dm_wcs(coadd_dim, pixel_scale=SCALE,
-                      big_coadd_dim_padding=3000,
-                      xoff=1000, yoff=450):
+def make_coadd_dm_wcs(
+    coadd_dim, pixel_scale=SCALE,
+    world_origin=WORLD_ORIGIN,
+    big_coadd_dim_padding=3000,
+    xoff=1000, yoff=450
+):
     """
     make a coadd wcs, using the default world origin.  Create
     a bbox within larger box
@@ -82,6 +85,9 @@ def make_coadd_dm_wcs(coadd_dim, pixel_scale=SCALE,
         Pixel dimension of the coadd image
     pixel_scale: float
         pixel scale
+    world_origin: galsim.CelestialCoord
+        sky coordinate of the world origin (it is the sky coordinate of the
+        center of large box)
     big_coadd_dim_padding: int
         padding for the larger coadd image. dim(big_coadd) = dim(coadd) + padding
     xoff: int
@@ -115,13 +121,17 @@ def make_coadd_dm_wcs(coadd_dim, pixel_scale=SCALE,
         make_wcs(
             scale=pixel_scale,
             image_origin=gs_coadd_origin,
-            world_origin=WORLD_ORIGIN,
+            world_origin=world_origin,
         )
     )
     return coadd_wcs, coadd_bbox
 
 
-def make_coadd_dm_wcs_simple(coadd_dim, pixel_scale=SCALE):
+def make_coadd_dm_wcs_simple(
+    coadd_dim,
+    pixel_scale=SCALE,
+    world_origin=WORLD_ORIGIN,
+):
     """
     make a coadd wcs without big coadd bbox padding, using the default world origin.
 
@@ -131,12 +141,18 @@ def make_coadd_dm_wcs_simple(coadd_dim, pixel_scale=SCALE):
         Pixel dimension of the coadd image
     pixel_scale: float
         pixel scale
+    world_origin: galsim.CelestialCoord
+        sky coordinate of the world origin (it is the sky coordinate of the
+        center of coadd boundary box)
 
     Returns
     --------
     (coadd dm wcs, coadd dm bbox), see make_dm_wcs for return wcs type
     """
 
-    return make_coadd_dm_wcs(coadd_dim, pixel_scale=pixel_scale,
-                             big_coadd_dim_padding=0,
-                             xoff=0, yoff=0)
+    return make_coadd_dm_wcs(
+        coadd_dim, pixel_scale=pixel_scale,
+        world_origin=WORLD_ORIGIN,
+        big_coadd_dim_padding=0,
+        xoff=0, yoff=0
+    )
diff --git a/descwl_shear_sims/wcs/sewcs.py b/descwl_shear_sims/wcs/sewcs.py
new file mode 100644
index 0000000..ef8140e
--- /dev/null
+++ b/descwl_shear_sims/wcs/sewcs.py
@@ -0,0 +1,65 @@
+import galsim
+import numpy as np
+
+from ..constants import SCALE, WORLD_ORIGIN
+from . import make_wcs
+
+
+def make_se_wcs(
+    *,
+    pixel_scale: float = SCALE,
+    image_origin: galsim.PositionD,
+    world_origin: galsim.CelestialCoord = WORLD_ORIGIN,
+    dither: bool = False,
+    rotate: bool = False,
+    theta: float | None = None,
+    rng=None,
+):
+    """
+    This function generates a single exposure galsim WCS
+
+    Parameters
+    ----------
+
+    pixel_scale: float
+        image pixel scale, default: 0.2
+    image_origin: galsim.PositionD
+        Image origin position
+    world_origin: galsim.CelestialCoord
+        Origin on the sky
+    dither: bool, optional
+        whether to do dither or not, default: False
+    rotate: bool
+        whether to do rotation or not, default: False
+    theta: float
+        rotation angle, optional, default: None
+    rng: numpy.random.RandomState
+        random number generator, optional, default: None
+
+    Returns
+    ----------
+    Galsim WCS of the single exposure
+    """
+
+    if dither:
+        # do a small offset of the origin
+        assert rng is not None
+        dither_range = 0.5
+        off = rng.uniform(low=-dither_range, high=dither_range, size=2)
+        offset = galsim.PositionD(x=off[0], y=off[1])
+        image_origin = image_origin + offset
+
+    if rotate:
+        # roate the single exposure
+        if theta is None:
+            assert rng is not None
+            theta = rng.uniform(low=0, high=2 * np.pi)
+    else:
+        theta = None
+
+    return make_wcs(
+        scale=pixel_scale,
+        theta=theta,
+        image_origin=image_origin,
+        world_origin=world_origin,
+    )
diff --git a/descwl_shear_sims/wcs/wcstools.py b/descwl_shear_sims/wcs/wcstools.py
index 435f915..a3b510d 100644
--- a/descwl_shear_sims/wcs/wcstools.py
+++ b/descwl_shear_sims/wcs/wcstools.py
@@ -60,7 +60,7 @@ def make_coadd_wcs(coadd_dim, pixel_scale=SCALE):
     --------
     A galsim wcs, see make_wcs for return type
     """
-    coadd_dims = [coadd_dim]*2
+    coadd_dims = [int(coadd_dim)]*2
     coadd_cen = (np.array(coadd_dims)-1)/2
     coadd_origin = galsim.PositionD(x=coadd_cen[1], y=coadd_cen[0])
     return make_wcs(